Which component failure could cause a vehicle to exhibit a LTFT of -20% and STFT of -10%?

A long-term fuel trim (LTFT) of -20% indicates that the engine's fuel system is compensating for an excessively rich running condition by reducing the amount of fuel injected over time. A short-term fuel trim (STFT) of -10% also suggests that the engine management system is making similar short-term adjustments.

In this context, a clogged Positive Crankcase Ventilation (PCV) valve can lead to increased crankcase pressure and can cause an imbalance in the air-fuel mixture. When the PCV valve is clogged, it can restrict the proper flow of gases, leading to an improper air-fuel mixture entering the engine. This can result in symptoms such as a rich fuel mixture, prompting the engine control module (ECM) to reduce the fuel injector pulse width, reflected in negative fuel trims.

The other components listed, while they can impact engine performance or air-fuel mixture in different ways, do not typically result in the specific fuel trim readings described. For example, a faulty mass airflow sensor can provide incorrect airflow data leading to fuel trim adjustments, but it would not usually result in such low values as indicated. A clogged fuel filter predominantly impacts fuel delivery and pressure rather than air-fuel mixture adjustments. Finally,